Dynamic compression device and driving tool

ABSTRACT

A bone screw has a spring section to allow the screw to elongate while it is being installed. The screw has a cavity with a proximal drive surface and a distal drive surface. The drive surfaces are located on opposite sides of the spring section. A drive tool that has proximal and distal drive members fits within the cavity. Rotating the drive tool transmits torque to both the proximal and distal drive sockets simultaneously.

FIELD OF THE INVENTION

This invention relates in general to fasteners for fastening portions ofa fractured bone together.

BACKGROUND OF THE INVENTION

One method of bone fracture repair comprises securing screws across thefractured portions of the bone. The purpose of the screw or screws is totightly secure the bone fragments together while the bone heals.Typically, the screws comprise lag design, each being a solid memberhaving an externally threaded section on one end and a head on the otherend. The head bears against the exterior surface of the bone when thescrew is secured to a desired torque to place the bone fragments incompression.

During the process of revascularization, the edges of the fracturecommonly will resorb, typically about 1 to 3 mm. This movement reducesor eliminates the initial compression that the screw has provided. Abone lag screw that could provide continuous dynamic compression acrossthe fracture while accommodating resorption as the bone heals wouldprovide improved long-term stabilization until revascularization iscompleted.

A number of patents disclose a spring section within a lag screw forthis purpose. The spring section would allow the screw to elongateslightly as it is being inserted. If sufficient initial compressionexisted, the spring section would provide dynamic compression during therepair process. The spring sections shown in these patents, however,could be damaged during insertion because the torque applied to the headof the screw passes through the spring section. This torque would resultin torsional deformation of the spring section during insertion, whichcould create stress risers within the spring that could lead to earlymechanical failure. Furthermore, during the process of removal, thespring could distort and fail mechanically, requiring extraction methodsthat could be more destructive to the bone.

SUMMARY OF THE INVENTION

In this invention, the dynamic compression device in the preferredembodiment is a screw having an elongated body with a distal end and aproximal end. The device has an external anchor section and a springsection located between the ends. A proximal drive surface is locatedproximal of the spring section. A distal drive surface is at a locationdistal of the spring section.

A drive tool is employed that has two spaced-apart drive members. One ofthe drive members engages the distal drive surface while the other drivemember engages the proximal drive surface. These two drive members applytorque simultaneously to the screw body on opposite ends of the springsection to assure that the spring section does not undergo torsionduring deployment.

In the preferred embodiment, the screw is hollow, and the distal drivesurface comprises a socket located within the cavity of the screw. Theproximal drive surface is also preferably a drive socket, and it islocated within a recess in the head. The tool has an elongated shankwith the distal drive member located on the tip. The proximal drivemember is slidably carried on the shank but rotates with the shank.

The screw of this invention has an enlarged head in the preferredembodiment, the head having a flange that is partially spherical. Awasher fits between the flange and the bone surface, and has one sidefor engaging the bone exterior. The interior of the washer comprises amating spherical surface for engaging the flange of the head. Thespherical surface allows the axis of the washer to incline relative tothe axis of the screw.

A stabilizing pin preferably fits within the cavity of the screw afterthe tool has been removed. The stabilizing pin extends through thespring section, reducing bending moments and protecting the spring.Also, the stabilizing pin retards bone ingrowth into the cavity.Reducing bone ingrowth makes the task of removing the screw easier ifthe screw is later removed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a screw constructed in accordance withthis invention.

FIG. 2 is a perspective view of a stabilizing pin for insertion into thecavity of the screw after deployment.

FIG. 3 is an enlarged view of a washer for use with the screw of FIG. 1.

FIG. 4 is a side elevational view of a tool constructed in accordancewith this invention for deploying the screw of FIG. 1.

FIG. 5 is a perspective view of the screw of FIG. 1.

FIG. 6 is a perspective view of the tool of FIG. 4.

FIG. 7 is a sectional view of the tool of FIG. 4, taken along the line7-7 of FIG. 4.

FIG. 8 is a sectional view of the screw of FIG. 1 with the tool of FIG.4 inserted.

FIG. 9 is a sectional view of the screw of FIG. 1 installed within abone and containing the stabilizing pin of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the fastener of this invention includes comprises ascrew 11 in the preferred embodiment that has an elongated body 13.Screw 11 may be formed of various biocompatible materials such asstainless steel or vitallium, but preferably is formed of titanium ortitanium alloy because of the enhanced fatigue properties.

Screw body 13 has an anchor section on its distal end, which preferablycomprises a set of external threads 15. Threads 15 may be a variety oftypes, but are shown as having a saw-tooth configuration with a sharpcrest 16. A cylindrical root 18 locates between adjacent crests 16. Theouter diameter of threads 15 measured at crests 16 is uniform, exceptfor the first thread 15 at the distal end, which is slightly smaller.The outer diameter of threads 15 is larger than the outer diameter ofbody 13. Threads 16 have a low pitch, a considerable dimension from root18 to crest 16, and resemble a flight of an auger. As shown in FIG. 5,one or two of the threads 15 at the proximal end may have back cutnotches or flutes 20 formed in them to facilitate re-cutting a threadedprofile in a bone if screw 11 is to be removed. Other types of anchorfeatures are also feasible for anchoring body 13 to an object byrotation. For example, helical flight segments that do not extend fullycircumferentially may be feasible in lieu of threads 15, particularlywhen the device is to be used other than as a bone screw.

As shown in FIG. 1, an enlarged head 17 is formed on the proximal end ofbody 13. Head 17 has a radially extending flange 19. The distal side offlange 19 is rounded, or partially spherical. A cavity 21 extendsaxially within body 13. Cavity 21 has a first portion 21 a (FIG. 1) thatis larger in diameter than the second portion 21 b. Second portion 21 bis located within threaded section 15. Cavity 21 extends completelythrough screw 11 from head 17 to the opposite end.

A helical slot 23 extends from the inner diameter of cavity portion 21 ato the outer diameter of body 13, defining a helical rib that forms aspring section 25. In this embodiment, slot 23 is located in a proximalportion of body 13, beginning near head 17 and extending downward abouthalf the length of cavity first portion 21 a. However, slot 23 could belocated at other points within body 13 between head 17 and threads 15.Spring section 25 will plastically deflect to allow axial elongation andcompression of body 13.

A distal drive surface 27 is located between threaded section 15 andspring section 25. Distal drive surface 27 is preferably a drive sockethaving a plurality of drive surfaces or flanks located within cavity 21at its junction with cavity second portion 21 b. Body 13 also has aproximal drive surface 29 located proximally of spring section 25.Preferably, proximal drive surface 29 comprises a drive socket formed inhead 17 and surrounded by flange 19. Alternately, proximal drive surface29 could be formed on the outer diameter of flange 19. Distal andproximal drive sockets 27, 29 could be of a variety of shapes forcausing rotation of screw 11, such as hex, splines or Torx. In theembodiment shown, distal drive socket 27 and proximal drive socket 29are shown as hex sockets. Proximal drive socket 29 has a larger diameterthan distal drive socket 27.

An internally threaded section 31 is optionally located in cavityportion 21 a between head 17 and spring section 25 for a purpose thatwill subsequently be explained. Referring to FIG. 3, a washer 33 ispreferably employed with screw 11. Washer 33 is an annular member ofbiocompatible material having a hole 35 through it that is partiallyspherical. Hole 35 is concave, providing a smaller inner diameter at itsdistal side 37 than its proximal side 39. This results in a concavesurface that is formed at the same contour as flange 19 for mating withflange 19. Distal side 37 may be slightly concave, as shown. Or ofvarying contours and textures

A tool 41, shown in FIGS. 4 and 6, is used to deploy screw 11 (FIG. 1).Tool 41 has a base 43 and a reduced diameter shank 45 extending from it.Shank 45 is a cylindrical member having a distal drive member 47integrally formed on its distal end. Shank 45 could be otherconfigurations rather than cylindrical, such as hexagonal. Drive member47 has an exterior surface contoured to mate with distal drive socket27. For illustration purposes, drive member 47 is shown as hexagonal inFIG. 4 and with a Torx configuration in FIG. 6.

Tool 41 also has a proximal drive member 49. Proximal drive member 49,as shown also in FIG. 7, is an annular member similar to a nut havingdrive flanks 50 on its exterior side that mate with proximal drivesocket 29. In the embodiment shown, drive flanks 50 are hexagonal. Ifscrew proximal drive surface 29 were alternately located on the exteriorof head 17, proximal drive member 49 would be a sleeve having driveflanks in its interior. Shank 45 of tool 41 has at least one groove 51that extends axially. A key 53 formed in the inner diameter of proximaldrive 49 engages groove 51 to cause proximal drive member 49 to rotatewith shank 45. In the preferred embodiment, there are two grooves 51spaced 180 degrees apart from each other and two keys 53. Rather thankey 53 and groove 51, other devices could be employed to cause proximaldrive member 49 to rotate with shank 45. For example, shank 45 and thepassage in drive member 49 could be polygonal.

Base 43 of tool 41 may comprise a handle for gripping by a user.Optionally, base 43 may have a polygonal drive head 55 formed on itsproximal end. In this embodiment, a conventional torque meter ormeasuring device 57 (FIG. 6) mounts to drive head 55. Torque meter 57will provide an indication of torque being transmitted by tool 41. Ahandle 59 is shown connected to the opposite end of torque meter 57 asshown in FIG. 6, for manually rotating tool 41. Torque meter 57 isoptional, and if desired, handle 59 could be coupled directly to drivehead 55.

As shown in FIG. 8, a series of indicia 61 is preferably formed on shank45. Indicia 61 comprises numbers or symbols calibrated to provide anindication to the user of the amount of deflection or compression ofscrew 11 during deployment. The alignment of the proximal side of screwhead 17 with a particular number or symbol of indicia 61 informs theoperator of the amount of elongation of spring section 25 that hasoccurred.

Referring to FIG. 2, a stabilizing pin 63 is optionally installed withincavity portion 21 a after removal of tool 41. Stabilizing pin 63 has anelongated cylindrical shank 65 and a threaded head 67. Head 67 securesto internal threads 31 in screw 11 (FIG. 1). Head has a drive surface,preferably a socket 68 with drive flanks for receiving a tool (notshown) to secure head 67 to threads 31. The tip of shank 65 locates atthe junction between cavity portion 21 a and distal drive socket 27. Theouter diameter of shank 65 is slightly less than the inner diameter ofcavity portion 21 a. Stabilizing pin 63 stiffens the construction toprevent bending of spring 25 and possible failure. Also, stabilizing pin63 serves to retard the ingrowth of bone through helical slot 23 intocavity 21 a. Rather than threads 67 engaging internally threaded section31, other types of retaining mechanisms could be employed, such as asnap ring or retainer ring.

The primary use for the preferred embodiment of screw 11 is as a bonefastener, but it may have other uses as well. FIG. 8 illustrates anexample of the primary use of the preferred embodiment. A bone 69 of apatient is shown having a fracture 71, resulting in separate objects orfragments 69 a and 69 b. To repair fracture 71, the operator or surgeonwill first insert a guide pin (not shown), or drill a pilot hole, fromfragment 69 a into fragment 69 b in a conventional manner to provide anaccurate axis of insertion of the device. The guide pin is a smalldiameter elongated member that has a threaded end that secures in thebase of the pilot hole. The operator then passes a reamer (not shown)over the guide pin to drill out a hole 73 in fragments 69 a and 69 b.The diameter of hole 73 is less than the diameter of screw threadedsection 15 at the distal end, but the same or slightly larger than thediameter of screw body 13 between threaded section 15 and head 17. Theoperator selects the depth of hole 73 to be longer than the length ofscrew 11 by an amount to accommodate the desired elongation of springsection 25. The operator then inserts a tool (not shown) with a threadedtap over the guide wire and into hole 73. The operator rotates the toolto form a set of helical grooves in hole 73 that match the pitch ofthreads 15. The operator removes the tap, but may leave the guide wirein place, if desired.

The operator slides washer 33 over screw body 13 and places its distalside 37 in contact with bone 69. Distal side 37 is preferably contouredto fit generally flush against the exterior portion of bone 69surrounding the entrance of hole 73. This exterior portion may begenerally convex, as shown, or other shapes. The operator inserts screw11 over the guide wire, if it is still in place, and into hole 73.Washer 33 will be in contact with the exterior surface of bone 69.

The operator engages tool distal drive member 47 with distal drivesocket 27. Simultaneously, the operator pushes proximal drive member 49axially along tool shank 45 until it fits within proximal drive socket29. The operator optionally attaches torque meter 57 (FIG. 6) and beginsrotation of tool 41. If remaining, the guide wire may extend through apassage provided in tool 41 and torque meter 57. The rotational force isexerted simultaneously against both drive sockets 27, 29. This rotationcauses external threads 15 to advance into the threaded profile of hole73. Because tool 41 drives both drive sockets 27, 29 simultaneously, notorque is applied to spring section 25.

As the operator continues rotating, screw flange 19 will engage washer33. The spherical surfaces 19 and 35 allow washer 33 to be angularlymisaligned up to a selected amount relative to head 17 due to irregularsurfaces of bone 69. That is, the axis of washer 33 may be at an acuteangle relative to the axis of screw 11. Also, the smooth sphericalsurfaces 19 and 35 reduce friction due to the rotation of screw 11 thatwould otherwise occur if head 17 directly contacted the exterior of bone69. Eventually, the contact of screw head 17 against washer 33 willcause washer 33 to bear tightly against the exterior surface of bone 69,preventing head 17 from any advancing further toward bone 69. Threadedsection 15, however, continues to advance into hole 73, causing theoverall length of screw 11 to elongate. The elongation takes place inspring section 25. The resiliency of spring section 25 creates a preloadforce against the exterior side of bone 69. The operator will continuerotating screw 11 until one of the indicia 61 indicates that the desiredelongation of screw 11 has taken place. The maximum elongation,typically about 3-6 millimeters, will be pre-calculated to avoid thetorque creating permanent axial deformation of spring section 25. Springsection 25 will thus remain elastic if not elongated past the maximumlimit. As a safety backup, the operator may monitor torque meter 57. Theamount of compressive load will be proportional to the elongation ofscrew 11.

Once the desired preload compression has been achieved, the operatorwill remove tool 41 and the guide wire if it is still in place, theninstalls stabilizing pin 63. The operator inserts a tool into socket 68(FIG. 2) in head 67 and tightens stabilizing pin 63 within internallythreaded section 31, as shown in FIG. 8. After stabilizing pin 63 hasbeen tightened, the operator removes the tool. Optionally, the operatormay coat the exterior of stabilizing pin 63 with bone wax to furtherretard ingrowth of bone into screw 11 and allow potential contraction ofspring section 25. Optionally, the operator could omit stabilizing pin63 to fill cavity 21 with bone wax or other material to retard ingrowthof bone into cavity 21 and facilitate later removal.

If screw 11 must later be removed, the process is reversed. The operatorinserts a tool into socket 68 (FIG. 2) of stabilizing pin 63 and removesstabilizing pin 63. The operator then engages tool 41 with screw 11,having engaged drive member 47 with drive socket 27 and drive member 49with drive socket 29, and rotates tool 41 in reverse. The back cutflutes 20 (FIG. 5) recut threads in bone 69 when tool 41 is rotated inreverse.

The invention has significant advantages. The spring section applies acontinuous compressive force to two objects, such as fragments of bone.The two separate drive surfaces on the opposite ends of the springsection avoid applying torque to the spring section during insertion orremoval, which would increase stress and possibly damage the springsection. Allowing the proximal drive member to move axially on the shankof the driving tool allows the distal drive member to remain inengagement with the distal drive socket while the screw elongates. Thewasher reduces rotary friction and allows compliance of the screw headso that the screw does not have to be precisely normal to the bonesurface. The stabilizing pin prevents cyclic bending to protect thedeployed spring section when used as a single device. Also, thestabilizing pin retards ingrowth into the screw, which otherwise wouldmake removal move difficult.

While the invention has been shown in only one of its forms, it shouldbe apparent to those skilled in the art that it is not so limited but itis susceptible to various changes without departing from the scope ofthe invention. For example, although shown for attaching fragments of abone, the device and tool could be used for fastening any objects thatdesire a compressive force to remain under dynamic conditions, includingindustrial applications.

1. An apparatus for fastening objects, comprising: an elongated bodyhaving a distal end and a proximal end; an external anchor section on adistal portion of the body for anchoring the distal portion of the bodyto an object; a spring section in the body at a location between theanchor section and the proximal end, the spring section allowing thebody to elongate from an initial length when being fastened in anobject, but urging the body toward the initial length; a proximal drivesurface at a location proximal of the spring section; and a distal drivesurface at a location distal of the spring section.
 2. The apparatusaccording to claim 1, further comprising: a partially spherical annularflange on the proximal end of the body; and a washer adapted to contactthe object, the washer having an interior shoulder that is partiallyspherical for mating engagement with the flange.
 3. The apparatusaccording to claim 1, wherein the proximal and distal drive surfacescomprise a plurality of circumferentially spaced drive flanks.
 4. Theapparatus according to claim 1, wherein the body is hollow and thespring section comprises a helical rib.
 5. The apparatus according toclaim 1, wherein the body is hollow, and the proximal and distal drivesurfaces comprise polygonal drive sockets formed within the body.
 6. Theapparatus according to claim 1, wherein the body is hollow, and theapparatus further comprises: a stabilizing pin that locates within thebody after the body is in full engagement with the object.
 7. Theapparatus according to claim 1, further comprising: a driving toolhaving a distal drive member that engages the distal drive surface and aproximal drive member that simultaneously engages the proximal drivesurface.
 8. The apparatus according to claim 1, further comprising: adriving tool having an elongated shank; a distal drive member located onthe shank that engages the distal drive surface; and a proximal drivemember located on the shank that simultaneously engages the proximaldrive surface, the proximal drive member being axially movable relativeto an axis of the shank.
 9. An apparatus for fastening objects,comprising: an elongated body having a distal end and a proximal end; anexternal threaded section on a distal portion of the body; a head formedon the proximal end of the body; a cavity extending through the headinto the body, defining a side wall with an interior side and anexterior side; a slot extending through the side wall from the interiorside to the exterior side, the slot extending helically along the bodyat a location between the threaded section and the head, defining aspring section; a proximal drive surface on the body at a locationproximal of the spring section; and a distal drive surface within thecavity at a location distal of the spring section, the proximal anddistal drive surfaces being positioned for receiving a tool thatsimultaneously engages both of the drive surfaces to rotate the threadedsection into engagement with an object without applying torque to thespring section.
 10. The apparatus according to claim 9, furthercomprising: a stabilizing pin placed in the cavity after the apparatushas been driven fully into engagement with the object, the stabilizingpin extending through the spring section.
 11. The apparatus according toclaim 9, further comprising: an internal threaded section in the cavityat a location proximal of the spring section; and a stabilizing pin thatis positioned in the cavity after the apparatus has been driven fullyinto engagement with the object, the stabilizing pin having an externalthreaded section that engages the internal threaded section in thecavity, the stabilizing pin having an elongated shank that extends pastthe spring section.
 12. The apparatus according to claim 9, wherein theproximal drive surface comprises a polygonal socket formed within thehead.
 13. The apparatus according to claim 9, wherein the cavity extendscompletely through the body from the distal end to the proximal end. 14.The apparatus according to claim 9, wherein the head has an annularflange that is a portion of a sphere; and wherein the apparatus furthercomprises: a washer adapted to contact the object, the washer having aninterior profile that is a portion of a sphere for mating engagementwith the flange of the head.
 15. An apparatus for fastening objects,comprising: an elongated body having a distal end and a proximal end; anexternal threaded section on a distal portion of the body; a head formedon the proximal end of the body and having an external flange; a cavityextending through the head into the body, defining a side wall with aninterior side and an exterior side; a slot extending through the sidewall from the interior side to the exterior side, the slot extendinghelically along the body at a location between the threaded section andthe head, defining a spring section that elongates when the apparatus isdeployed; a proximal drive socket located within the head; a distaldrive socket located within the cavity distal of the spring section; adriving tool having an elongated shank that inserts into the cavity tosecure the fastener to an object; a distal drive member located on adistal end of the shank, the distal drive member engaging the distaldrive socket; and a proximal drive member carried by the shank for axialsliding movement relative to an axis of the shank for simultaneouslyengaging the proximal drive socket while the distal drive member engagesthe distal drive socket and the spring section elongates, the proximaldrive member being rotatable with the shank.
 16. The apparatus accordingto claim 15, further comprising: an internal threaded section in thecavity at a location proximal of the spring section; and a stabilizingpin that is positioned in the cavity after the driving tool has beenremoved, the stabilizing pin having an external threaded section thatengages the internal threaded section in the cavity, the stabilizing pinhaving an elongated shank that extends past the spring section.
 17. Theapparatus according to claim 15, wherein the flange has a sphericalportion; and wherein the apparatus further comprises: a washer adaptedto contact the object, the washer having an interior profile that has aspherical portion for mating engagement with the flange of the head. 18.The apparatus according to claim 15, further comprising a set of indiciaon the shank adjacent the proximal drive member for providing anindication of elongation of the spring section.
 19. An apparatus forfastening objects, comprising: a driving tool having an elongated shank;a distal drive member located on a distal end of the shank, the distaldrive member having polygonal drive flanks for engaging and rotating adistal drive socket of a fastener; and a proximal drive member carriedby the shank for axial sliding movement relative to an axis of theshank, the proximal drive member having polygonal drive flanks and beingrotatable with the shank for simultaneously engaging and rotating aproximal drive socket of the fastener.
 20. A method of fastening a firstobject to a second object, comprising: (a) forming a hole through thefirst object and into the second object; (b) providing a fastener withan external anchor section, a spring section, a proximal drive surfaceat a location proximal of the spring section, and a distal drive surfaceat a location distal of the spring section; (c) providing a tool with adistal drive member and a proximal drive member; and (d) engaging thedistal drive member with the distal drive surface and the proximal drivemember with the proximal drive surface, inserting the fastener into thehole, and simultaneously rotating the distal and proximal drive membersto secure the anchor section in a portion of the hole in the secondobject.
 21. The method according to claim 20, wherein: step (b)comprises forming the proximal and distal drive surfaces within a cavityof the fastener; and step (d) comprises inserting the distal andproximal drive members into the cavity.
 22. The method according toclaim 20, further comprising: after step (d), withdrawing the tool andsecuring a stabilizing pin within the cavity.